托福阅读真题Official 47 Passage 3（五）

托福阅读真题Official 47 Passage 3（五）

Coral Reefs

An important environment that is more or less totally restricted to the intertropical zone is the coral reef.Coral reefs are found where the ocean water temperature is not less than 21°C,where there is a firm substratum,and where the seawater is not rendered too dark by excessive amounts of river-borne sediment.They will not grow in very deep water,so a platform within 30 to 40 meters of the surface is a necessary prerequisite for their development.Their physical structure is dominated by the skeletons of corals,which are carnivorous animals living off zooplankton.However,in addition to corals there are enormous quantities of algae,some calcareous,which help to build the reefs.The size of reefs is variable.Some atolls are very large—Kwajalein in the Marshall Islands of the South Pacific is 120 kilometers long and as much as 24 kilometers across—but most are very much smaller,and rise only a few meters above the water.The 2,000 kilometer complex of reefs known as the Great Barrier Reef,which forms a gigantic natural breakwater off the northeast coast of Australia,is by far the greatest coral structure on Earth.

Coral reefs have fascinated scientists for almost 200 years,and some of the most pertinent observations of them were made in the 1830s by Charles Darwin on the voyage of the Beagle.He recognized that there were three major kinds:fringing reefs,barrier reefs,and atolls;and he saw that they were related to each other in a logical and gradational sequence.A fringing reef is one that lies close to the shore of some continent or island.Its surface forms an uneven and rather rough platform around the coast,about the level of low water,and its outer edge slopes downwards into the sea.Between the fringing reef and the land there is sometimes a small channel or lagoon.When the lagoon is wide and deep and the reef lies at some distance from the shore and rises from deep water it is called a barrier reef.An atoll is a reef in the form of a ring or horseshoe with a lagoon in the center.

Darwin’s theory was that the succession from one coral reef type to another could be achieved by the upward growth of coral from a sinking platform,and that there would be a progression from a fringing reef,through the barrier reef stage until,with the disappearance through subsidence(sinking)of the central island,only a reef-enclosed lagoon or atoll would survive.A long time after Darwin put forward this theory,some deep boreholes were drilled in the Pacific atolls in the 1950s.The drill holes passed through more than a thousand meters of coral before reaching the rock substratum of the ocean floor,and indicated that the coral had been growing upward for tens of millions of years as Earth’s crust subsided at a rate of between 15 and 51 meters per million years.Darwin’s theory was therefore proved basically correct.There are some submarine islands called guyots and seamounts,in which subsidence associated with sea-floor spreading has been too speedy for coral growth to keep up.

Like mangrove swamps,coral reefs are extremely important habitats.Their diversity of coral genera is greatest in the warm waters of the Indian Ocean and the western Pacific.Indeed,they have been called the marine version of the tropical rain forest,rivaling their terrestrial counterparts in both richness of species and biological productivity.They also have significance because they provide coastal protection,opportunities for recreation,and are potential sources of substances like medicinal drugs.At present they are coming under a variety of threats,of which two of the most important are dredging and the effects of increased siltation brought about by accelerated erosion from neighboring land areas.

Question 9 of 14

Which of the following can be inferred from paragraph 3 about the Pacific atolls?

A.They were once fringing reefs around the coasts of islands.

B.They were first observed by Darwin during his voyage on the Beagle.

C.They will eventually become fringing reefs.

D.They are located where the ocean floor does not sink.

正确答案：A

题目详解

题型分类：推论题

原文定位：根据the Pacific atolls定位到第2、3句A long time after Darwin put forward this theory,some deep boreholes were drilled in the Pacific atolls in the 1950s...and indicated that the coral...growing upward...as Earth's crust subsided...Darwin s theory was therefore proved basically correct。（所在句原文高亮）

选项分析：

第二句提到人们在太平洋的环礁上钻了一些深孔，最后一句说明达尔文的理论基本上是正确的，从上文找答案，推测出环礁是暗礁演变来的，也就是说他们曾经是暗礁，对应原文atoll would survive，因此推理到选项A正确。

B选项，本段没有提到比格尔航行，无中生有。

C选项，定位区域没有提到环礁会最终变成fringing reefs，无中生有。

D选项，本段没有环礁的位置，无中生有。

Question 10 of 14

Why does the passage provide the information that the drill holes in the Pacific atolls passed through more than a thousand meters of coral before reaching the rock substratum of the ocean floor?

A.To emphasize that according to Darwin’s view coral can grow at great depths

B.To indicate how scientists knew the rate at which Earth’s crust had subsided

C.To support the claim that coral reefs take millions of years to form

D.To present the evidence that confirmed Darwin’s account of coral reef evolution

正确答案：D

题目详解

题型分类：修辞目的题

题干分析：考察句子之间的关系

选项分析：

根据题干The drill holes定位到最后2句话the drill holes passed through...Darwin s theory was therefore proved basically correct原文举例太平洋环礁，就是为了证明达尔文理论是正确的，通过例子得出结论，即选项D正确。

A选项，原文倒数第三句中说珊瑚可以生长for tens of millions of years，并没有提到珊瑚可以在很深的地方生长，与原文不符。

B选项，原文倒数第三句说地壳每一百万年15到51米的速度在下沉subsided at a rate of between 15 and 51 meters per million years，但并没有提到科学家是如何知道的，与原文不符。

C选项，没有提到珊瑚形成需要数百万年，无中生有。